1. Field of the Invention
The present invention relates to an image writing apparatus. More particularly, the invention relates to an image writing apparatus suitable for use as a color forming apparatus such as a color laser beam printer, a color digital copying machine, or a multi-function printer.
2. Description of the Related Art
In scanning optical apparatuses such as laser beam printers, color digital copying machines or multi-function printers, it is the conventional practice to record an image by periodically causing deflection of a beam, which is optically modulated or emitted in response to an image signal from a light source, by using an optical deflector comprising, for example, a rotating polygon mirror, and conveying the deflected beam into a spot on the surface of a photosensitive recording medium (photosensitive drum) by a scanning lens system (scanning optical system) having an fθ characteristic, to optically scan the surface of said recording medium.
FIG. 15 is a partial schematic view of a conventional optical scanning apparatus. In FIG. 15, a diverging beam emitted from a light source 91 comprising a semiconductor laser and the like is converted by a collimator lens 92 into a substantially parallel beam (or a converging beam). The beam (light intensity) is shaped by an aperture stop 93, and is incident on a cylindrical lens 94 having a refractive strength only in the sub-scanning direction. The beam incident on the cylindrical lens 94 is emitted unrefracted in the main scanning cross-section, and in the sub-scanning cross-section, it converges and is formed substantially into a line image near a deflecting surface 95a of the light deflector 95 comprising a rotating polygon mirror.
Subsequently, the beam reflected and deflected at the deflecting surface 95a of the light deflector 95 is introduced onto the surface of the photosensitive drum 98, serving as the scanned surface via a reflecting mirror 97 through a scanning lens system (scanning optical system) 96 having an fθ characteristic, and the deflector 95 is rotated in the direction indicated by arrow A, thereby optically scanning the photosensitive drum 98 surface in the direction indicated by arrow B (main scanning direction) at a constant speed to record image information.
Various color image forming apparatuses, such as color laser beam printers, color digital copying machines, and multi-function printers, each having a plurality of image carriers corresponding to the primary colors of output images (yellow: Y, magenta: M, cyan: C, and black: Bk) have conventionally been used. In these conventional color image forming apparatuses, each image carrier has a configuration including a plurality of optical scanning apparatuses like the above-mentioned conventional case are arranged, or a plurality of optical scanning apparatuses capable of scanning a plurality of image carriers simultaneously.
For example, in Japanese Patent Laid-Open No. 8-50385, four photosensitive drums are arranged, these drums serving as four image carriers corresponding to the colors yellow, magenta, cyan, and black, and one optical scanning apparatus is provided for each photosensitive drum. A desired image is formed by overlapping images of the individual colors on a conveyor belt.
In Japanese Patent Laid-Open No. 6-18796, a desired color image is formed by arranging two optical scanning apparatuses capable of scanning two photosensitive drums from among the four photosensitive drums serving as four image carriers corresponding to the four colors.
When forming a color image by using a plurality of optical scanning apparatuses, the positions of spots (dots) forming an image on the image carriers by the operation of the individual optical scanning apparatuses should be relatively aligned in each scanning region, both in the main scanning direction and in the sub-scanning direction. This means that it is necessary that the spot intervals in the main scanning direction and the scanning line inclination or curvature and the line intervals in the sub-scanning direction be in agreement. If this relative alignment of the spot positions is not achieved, upon overlapping on the conveyor belt., a color shift causes a decrease in quality of the output image. It is therefore important that the scanning accuracy of the individual optical scanning apparatuses be uniform and the positional relationship between the optical scanning apparatus and the corresponding image carrier be in agreement in each case.
For example, in Japanese Patent Laid-Open No. 8-50385, a color image is formed by using four optical scanning apparatuses and four image carriers corresponding thereto. In this case, it is desirable that dots formed on the image carrier by the optical scanning apparatus be perfectly in alignment with the corresponding dots formed by the three other optical scanning apparatuses when they overlap on the conveyor belt.
In an actual optical scanning apparatus, however, shifts between these dot positions are caused by accuracy errors of the individual optical parts, accuracy errors of the individual mechanical components for assembling the optical parts, such as an optical box, assembly errors of the optical parts, and relative positional errors between the optical scanning apparatus and the image carrier. When all of the optical scanning apparatuses have identical errors, no shift in the dot position is produced. Usually, however, the plurality of optical scanning apparatuses have different errors, and this is a factor causing color shift. The resultant color image therefore exhibits color shift in the main scanning direction as well as in the sub-scanning direction.
Color shifts in the sub-scanning direction caused by optical factors can be broadly classified into a scanning line inclination component and a curvature factor. In the optical scanning apparatus shown in FIG. 15, for example, when the optical scanning apparatus has a single-unit accuracy error caused by a slight rotation of the optical surface of the scanning lens system 96, or when a lens is wrongly mounted in the optical casing tilting in the axial direction in parallel with the optical axis occurs due to this assembly error, and the dot positions in the sub-scanning direction having an inclination component, as shown in FIG. 16. This phenomenon will now be described for two color shift components from among the four colors for simplicity of explanation.
On the assumption that one component is for cyan (C) and the other is for magenta (M) in FIG. 16, scanning line inclination is produced for both of these colors. The amount of inclination differs between the two colors in FIG. 16, because there are variations in the error components, such as a single-unit accuracy error and an assembly error. If both cyan and magenta have identical error components, i.e., if the two colors, having errors as compared with an ideal state, have the same amounts of inclination when there is no dispersion, it is possible to obtain an image free from color shift.
Actually, however, there are variations in all error factors, and this causes color shifting. To reduce this color shifting, in the conventional optical scanning apparatus, an adjusting mechanism is provided, for example, to tilt the entire optical scanning apparatus to make an adjustment so as to achieve perfect positional alignment of the scanning lines.
When an inclination of the scanning lines shown in FIG. 16 occurs, the inclination components of two colors finally agree with each other as shown in FIG. 17, by adjusting the inclination for the cyan and magenta optical scanning apparatuses around an axis in parallel with the optical axes of these optical scanning apparatus, thus correcting the color shifting.
When the lens has a single-unit accuracy error, or when the lens is mounted so as to be tilted or shifted in the axial direction in parallel with the main scanning direction due to an assembly error in the optical casing, the dot position in the sub-scanning direction would have a curvature component of the scanning lines, as shown in FIG. 18.
As in FIG. 16, FIG. 18 assumes that one curvature is for cyan (C), and the other is for magenta (M), and curvature of the scanning line curvature is produced for both. In FIG. 18, the amount of curvature differs between the two colors because there are variations in the error components, such as single-unit accuracy error and assembly error. When cyan and magenta have identical error components, i.e., when they deviate from the ideal state, but there is no dispersion in this state, and the same amounts of curvature result, a color image free from color shift is obtained.
Actually, however, all the error factors contain variations, causing color shift. In order to reduce the color shift, an adjusting mechanism is provided in conventional optical scanning apparatuses. For example, in Japanese Patent Laid-Open No. 2000-258713, the scanning line curvature is adjusted for by bending the reflecting mirror of the optical scanning apparatus.
When a scanning line curvature as shown in FIG. 18 occurs, adjustment is made by bending the mirrors provided in the optical scanning apparatus for cyan and magenta. In this adjustment method, however, it is necessary to bend the mirror itself by applying a large stress because of the low sensitivity of the mirror to scanning line curvature, and assembly accuracy, environmental changes, and the adjustment accuracy itself are problematic.
One of the most important factors causing scanning line curvature is the single-unit accuracy error of the lens. At present, the scanning lenses used in scanning optical apparatuses are formed in a plastic mold or a glass mold in response to the demands for cost reduction. Supply of low-cost lenses is therefore permitted by manufacturing a die having a plurality of cavities with identical specifications, or preparing a plurality of dies with identical specifications.
In FIG. 19, for example, four lenses with identical optical characteristics can be manufactured in a forming step by providing four cavities A, B, C and D in a single mold Z. In FIG. 20, it is possible to manufacture four lenses during the forming period of a production run by providing four molds V, W, X and Y having cavities E, F, G and H with identical specifications.
However, in the mold Z of FIG. 19, for example, frames and parts forming the cavities A, B, C and D having identical optical characteristics contain manufacturing errors including assembly errors with the mold. Because there occur delicate differences in the forming conditions between cavities, these factors cause accuracy errors in the four lenses formed. This is also the case with the configuration shown in FIG. 20.
The face apex height in the sub-scanning direction of the thus formed four lenses A, B, C and D may have single-unit accuracy errors, such as a curvature, along the main scanning direction, as shown in FIG. 21, and the single-unit accuracy error may often differ for the four lenses A, B, C and D. The occurrence of such single-unit accuracy errors causes refraction of a beam passing through the lens in the sub-scanning direction, and would cause scanning line curvature on the scanned surface. This scanning line curvature is caused by the four lenses A, B, C and D, and the scanning line curvature differs between the four lenses depending on the single-unit accuracy error. When four such lenses A, B, C and D are arranged in a single color image forming apparatus, this is a factor causing color shift in the sub-scanning direction.
The single-unit accuracy error of a lens is similarly a factor causing a partial magnification error in the main scanning direction. For example, when a geometric surface error from the design shape of the second face (emitting face) of the lens is as shown by a thick line in FIG. 22, and upon drawing dots at equal intervals on the scanned surface, there is an interval error as compared with an ideal interval, i.e., a partial magnification error. The partial magnification error differs between the four lenses A, B, C and D. Arrangement of four such lenses A, B, C and D in a single color image forming apparatus is a factor causing color shift in the main scanning direction.